The problem of amplifying optical signals for long distance transmission was successfully addressed by the development of Erbium doped fiber amplifiers (EDFAs). An EDFA consists of a length of silica fiber with the core doped with ionized atoms (Er3+) of the rare earth element Erbium. The fiber is pumped with a laser at a wavelength of 980 nm or 1480 nm. The doped, pumped fiber is optically coupled with the transmission fiber so that the input signal is combined with the pump signal in the doped fiber. Early EDFAs could provide 30 to 40 dB of gain in C-band extending between 1530 to 1565 nm with noise figures of less than 5 dB. Recently, EDFAs have been developed that can provide similar performance in the L-band (1565 to 1625 nm).
There is great interest in the telecommunications industry to make use of the optical spectrum range with wavelengths shorter than those currently achievable with conventional C-band and L-band EDFAs. This wavelength range, commonly called the “S-band” or “short-band” is considered to cover wavelengths between about 1425 nm and about 1525 nm. The gain in the S-band typically observed in EDFAs is limited by several factors, including incomplete inversion of the active erbium ions and by amplified spontaneous emissions (ASE) or lasing from the high gain peak near 1530 nm.
In producing an EDFA for the S-band the relatively high losses and low gains over the S-band render the selection of fiber and the design of the amplifier system very difficult. In fact, the problems are so severe that the prior art teaches interposition of external filters between EDFA sections to produce an S-band EDFA. For example, Ishikawa et al. disclose a method of fabricating an S-band EDFA by cascading five stages of silica-based EDFA and four ASE suppressing filters in Ishikawa et al., “Novel 1500 nm-Band EDFA with discrete Raman Amplifier”, ECOC-2001, Post Deadline Paper. In Ishikawa et al.'s experimental setup, the length of each EDFA is 4.5 meters. The absorption of each suppressing filter at 1.53 μm is about 30 dB and the insertion losses of each suppressing filter at 1.48 μm and 0.98 μm are about 2 dB and 1 dB respectively. The pumping configuration is bi-directional, using a 0.98 μm wavelength to keep a high inversion of more than D≧0.7 (D, relative inversion). The forward and backward pumping powers are the same and the total pumping power is 480 mW. Ishikawa et al. show a maximum gain of 25 dB at 1518.7 nm with 9 dB gain tilt.
This method is relatively complicated and not cost-effective, as it requires five EDFAs, four ASE suppressing filters and high pump power. Also, each of the ASE suppressing filters used in Ishikawa et al.'s method introduces an additional insertion loss of 1-2 dB. The total additional insertion loss is thus about 4-8 dB.
Another approach to providing amplification in the S-band has focused on fiber amplifiers using Thulium as the lasing medium doped into a Fluoride fiber core (TDFAs). See, for example, “Gain-Shifted Dual-Wavelength-Pumped Thulium-Doped-Fiber Amplifier for WDM Signals in the 1.48-1.51-μm Wavelength Region” by Tadashi Kasamatsu, et. al., in IEEE Photonics Technology Letters, Vol. 13, No. 1, January 2001, pg. 31-33 and references therein. While good optical performance has been obtained using TDFAs, this performance has only been possible using complex, non-standard and/or expensive pumping schemes. Also, TDFAs suffer from the problems inherent to their Fluoride fiber host material, namely high fiber cost, poor reliability and difficulty splicing to standard silica fibers used elsewhere in the amplifier system.
Optical amplifiers (such as EDFAs, TDFAs, Raman amplifiers, semiconductor amplifiers, etc . . . ) are used for several purposes in a telecommunications network. The most important use is to compensate for span loss (transmission fiber loss accumulated over tens or hundreds of km), in which case the amplifier is typically called an “in-line amplifier”. In-line amplifiers must provide a small-to-moderate amount of optical power per optical channel (typically 0.1-10 mW), but must also exhibit low noise figure and good gain flatness in the case of WDM networks. The latter two requirements result from the accumulated effects of a long cascade of amplifiers over a long fiber link of hundreds to thousands of km in length.
Optical amplifiers are also used as pre-amplifiers. Pre-amplifiers are typically used in order to improve the sensitivity of receivers, in ways that are well known in the art. Typically, the pre-amplifier is located just before the signal receiver in order to increase the signal strength (optical power) to a level well above the (electronic, or thermal) detector noise. Pre-amplifiers should exhibit very good noise figure, though they do not need to operate at high powers or with flattened gain profiles because they typically are used to amplify one or a small number of optical channels.
Optical amplifiers are also used as power-amplifiers. Power amplifiers are used in ways well known in the art to provide high optical power. Typically, they are operated with relatively high input signal strengths (i.e. are saturated) with good input signal-to-noise ratios, and therefore do not need very good noise figures. Also, they typically do not need very high gain. Power amplifiers are used when a large number of WDM channels are present, even when, for example, each channel needs only a moderate level of power. Power amplifiers are also used preceding long/lossy links in order to pre-compensate for the upcoming losses.
Finally, optical amplifiers are used for many other applications inside communications networks. Some examples are: power boosting prior-to or after splitting a signal into many parallel outputs, compensating for lossy network modules such as cross-connects and switches, providing high enough optical powers to pump nonlinear devices such as optically driven optical switches or optical wavelength converters.
In view of the above, it would be an advance in the art to provide an optical communication system that can take advantage of EDFAs for amplifying signals in the S-band. In particular, it would be advantageous to provide low-cost S-band EDFAs for use in such communication systems to achieve low-cost pre-amplification, power-boosting and in-line amplification.